A man uses a cellphone as he passes a T-Mobile store in New York, Sept. 12, 2012. Credit reporting agency Experian on Thursday said that hackers accessed the social security numbers, birthdates and other personal information belonging to about 15 million T-Mobile wireless customers. T-Mobile uses Experian to check the credit of its customers.

A security breach has exposed 15 million T-Mobile customers’ data to hackers, including addresses, birthdates, personal information, and Social Security numbers.

In a massive security breach of credit reporting agency Experian, which T-Mobile uses to run credit checks on potential customers who have applied for a phone plan or financing, has resulted in stolen data for many T-Mobile customers. Any T-Mobile customers who applied between September 2013 to September 2015 may have had their Social Security numbers and other information stolen, Experian said Thursday.

T-Mobile chief executive John Legere was candid and frank about his frustration over the incident. “Obviously I am incredibly mad about this data breach,' ” he told USA Today.

The hack is still being investigated. In a statement posted on the company website, Experian said that it is taking steps to prevent further breaches and is offering T-Mobile customers two free years of credit monitoring services at ProtectMyID.com, which Experian owns.

Customers, however, are thinking twice about using a credit monitoring service from the company that just exposed 15 million users’ data. Gartner security analyst Avivah Litan told the Associated Press, “customers will be cynical.... Why would you trust someone with your accounts that’s been breached.”

Many customers expressed similar concerns in direct tweets to Mr. Legere, who responded Thursday evening that he was working to find an alternative option for customers.

I hear you re: Experian as service protection option. I am moving as fast as possible to get an alternate option in place by tomorrow.

T-Mobile and Experian have reported that payment card and banking information is still safe and not affected by the breach. The loss of Social Security number information is another matter. Stolen Social Security numbers can be used to open up new financial accounts and to finance loans.

“There are more steps you have to take to minimize your risk when your Social Security number is compromised,” Eva Velasquez, the CEO of the Identity Theft Resource Center, told AP.

Experian is not the first company to experience a security breach, even one of this magnitude. According to the Identity Theft Resource Center, almost 800 data breaches were reported by US organizations last year. Just last month, the United States released information that the amount of biometric data stolen in the Office of Personal Management was five times more than previously estimated.